The present invention relates generally to techniques of fastening permanent magnet structures to ferromagnetic bodies in a manner to minimize flux short circuiting by the body while maintaining the permanent magnet structure rigidly in an accurately determined location. More particularly the present invention relates to such techniques where the body is a flywheel of a small internal combustion engine and the magnet structure is fastened near the outer periphery thereof as the moving portion of the engine ignition system.
Ignition systems for small internal combustion engines employing a permanent magnet rotating with the engine flywheel and a stator structure positioned either radially outwardly or radially inwardly of the magnet to have the flux patterns therein periodically changed by passage of the magnet are old and well known in the internal combustion engine art. Such ignition systems frequently employ two or three stator legs in close proximity to the path of the magnet and may rely on a capacitor discharge technique or solid state triggering schemes to induce a high voltage in a secondary winding of an ignition coil for ignition spark generating purposes. While forming no part of the present invention, it is contemplated that the pair of magnetic poles of the magnet group will sweep past an external E-shaped stator of an ignition system employing solid state techniques without mechanical breaker points of a type in current commercial use by applicant's assignee, however, it will be clear that the techniques of the present invention are applicable to a wide variety of ignition systems, battery charging schemes and other applications where it is desired to mount a permanent magnet on a ferromagnetic body in a magnetically isolated fashion.
Many small internal combustion engines employ a flywheel fabricated of cast aluminum and with such non-magnetic flywheel materials it has been a common technique to merely form a magnet group receiving pocket within the aluminum casting and then to fix the magnet group within that pocket by a pair of roll pins. U.S. Pat. No. 4,179,634 has addressed the problem of mounting such magnet group on a flywheel of either a non-magnetic or ferromagnetic nature and suggests a not altogether satisfactory solution to the magnetic flux short circuiting problems associated with a flywheel of a ferrous material. This U.S. patent suggests a nonferrous insert having a cavity within which the magnet group resides as illustrated in FIGS. 7 and 8 thereof. In those drawing figures, the magnet group is fastened within the nonferrous insert employing the standard technique of roll pins. The nonferrous pocket is in turn fastened by screws to the flywheel. As there is always some clearance between the screws and the nonthreaded member through which those screws pass, the positioning of the nonferrous pocket is necessarily somewhat inaccurate in turn creating problems of variable air gap between the magnet structure on the flywheel and the fixed stator structure adjacent thereto. Also, typically, the region occupied by the magnet group detracts from the remaining annular region frequently occupied by air circulating fins for engine cooling purposes. Thus the more substantial angular space occupied by the nonferrous pocket in this patented arrangement detracts from the cooling of the engine. A still further drawback of this arrangement is of course the number and complexity of parts employed.